Vacuum may be used in a vehicle to apply motive force in vehicle systems. For example, vacuum may be used to apply vehicle brakes, operate a turbocharger waste gate, and adjust valve positions in heating and ventilation ducts. However, vacuum in vehicle systems is becoming a less available resource due to the trend of engine downsizing and variable valve timing to improve vehicle fuel economy.
One of the more significant consumers of vacuum in a vehicle is the vehicle brake system. Vacuum is used in a brake booster to apply brakes. In particular, vacuum is applied to both sides of a brake booster diaphragm when brakes are not applied. Pressure equalization across the diaphragm allows the diaphragm to return to a position where a piston in the master cylinder does not increase brake line pressure. When the brakes are applied, vacuum on a working side of the diaphragm is displaced with ambient air while vacuum remains present on the vacuum side of the diaphragm. Consequently, a pressure differential is produced across the diaphragm that motivates the diaphragm to apply force to the piston in the master cylinder, thereby increasing brake pressure and applying the brakes.
During vehicle braking, a driver receives visual and physical cues that allow the driver to know whether or not a proper amount of force is being applied to the brake pedal to provide the desired braking amount or level. However, when the vehicle is stopped, the driver receives much less information regarding whether or not braking force is adequate or more than is desired to keep the vehicle from moving. Consequently, the driver may apply more brake force than is desired to keep the vehicle from moving. As a result, more vacuum than is desired may be consumed when the vehicle is stopped.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for conserving vacuum, comprising: limiting a brake line pressure increase at a wheel brake in response to an increasing in brake pedal force when a vehicle is stopped.
By limiting a brake line pressure increase, it may be possible to reduce vacuum consumption in a vehicle. Specifically, pressure in brake lines supplying wheel brakes may be limited via closing a valve located between a master cylinder piston and wheel brakes. Closing the valve limits master cylinder piston motion because brake fluid between the master cylinder piston and valve is nearly incompressible, thereby limiting master cylinder piston motion when the valve is closed and the brake is applied. The master cylinder piston is also mechanically coupled to a diaphragm in the brake booster that separates a brake booster working chamber from a brake booster vacuum chamber. Consequently, brake booster diaphragm motion is limited when master cylinder piston motion is limited. The brake booster diaphragm defines one side of the brake booster working chamber, and brake booster working chamber volume is substantially fixed (e.g., changes by less than 10% of total brake booster volume) when motion of the diaphragm is limited via the master cylinder piston. As a result, a driver may only decrease vacuum in the working chamber to an extent determined by the volume of the brake booster working chamber, which is related to the brake booster diaphragm position. In this way, vehicle brakes may be applied to provide a desired amount of braking force while brake booster vacuum consumption is limited.
The present description may provide several advantages. In particular, the approach may conserve vacuum in a vehicle so that the vehicle's engine operates for less time at low intake manifold pressures. The approach may also conserve fuel since the engine may be able to operate more efficiently at higher intake manifold pressures for longer periods of time. Additionally, the approach conserves vacuum responsive to vehicle operating conditions such as road grade and vehicle mass.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.